In many modern reproduction and printing techniques, e.g., electrophotography, electrostatography and magnetography, a powder image is formed on a re-usable image registration medium, by means of toner powder containing thermoplastic resin. Generally the powder image is then transferred and fixed onto a receiving material, such as ordinary paper. The transfer of the powder image to the receiving material can be achieved by bringing the powder image registration medium into pressure contact with a fixing member, the surface of Which consists of silicone rubber having a low affinity for the softened powder. The powder image transferred onto the fixing member as a result of the pressure exerted is then transferred by pressure to the receiving material. The powder is softened by heating before and/or during passage through the pressure zone so that it becomes so viscous that it forms a cohesive layer in response to the exerted pressure. This layer penetrates at least partially into the receiving material. After cooling, the image is permanently bonded to the image receiving material.
Heating of the powder image is effected by heating the fixing member, which functions as a temporary receiving support for the powder image and possibly the receiving material itself. Image transfer processes of this kind are described, inter alia, in U.S. Pat. Nos. 3,554,836 and 3,893,761 and UK Patent No. 1 279 687.
Image-forming processes are also known in which the powder image is transferred from the image registration medium directly, e.g., in response to an electric field, to the receiving material. It is then fixed by passing the receiving material through a pressure zone of two cooperating fixing members. The fixing member which presses against the image is typically provided with a layer of silicone rubber. At least one of the fixing members is heated to soften the powder image. Since both fixing members may come into contact with the thermoplastic resin, e.g., when the image receiving material is printed on both sides, it is usual for both fixing members to be provided with an outer covering of silicone rubber.
RTV or HTV silicone rubbers are normally used as surface covering for fixing members. They are obtained by cross-linking into an elastomeric composition polyorganosiloxane mixtures which bear reactive groups, under the influence of a suitable catalyst, at room temperature or at elevated temperature. The silicone rubber covering may also contain additives to improve its properties such as mechanical strength, thermal conductivity and antistatic behavior. Typical additives are fillers such as silica, titanium dioxide, calcium carbonate, iron oxide, teflon and carbon. The amount of additive in the covering may be 60% by weight or even more and is often between 40 and 50% by weight. Typical rubber compositions for forming a covering for fixing members which can be used as a temporary support for a powder image are described in UK patent No. 1 279 687 and Example 1 of European Patent Application No. 146 980. See also, British Patent Application No. 2 081 646 and European Application No. 0 062 048. In the former case an addition polymerization silicone rubber from low molecular (10 or less monomeric units) polyorganosiloxane is used as the bearing surface of the fusing member. Each Si-atom in the molecule includes a vinyl or allyl group. The latter patent application teaches an addition type silicone rubber bearing surface.
In addition to good mechanical strength required for long life, the rubber coverings present on fixing members should have a wide working range, i.e., there should be a wide temperature range within which the softened toner powder does not stick to the silicone rubber covering on leaving the pressure zone in which the powder image is fixed on the receiving material. This working range must also be situated at a temperature level at which sufficient fixing of the toner powder on the receiving material is obtained in the pressure zone. The working range of the silicone rubber coverings gradually reduces during operation of the fixing devices, in particular, the top limit of the working range decreases. Finally a point is reached in which there is no practical working range, and the fixing member has to be replaced. The development of everfaster copying machines and the continual search for components having a longer life mean that there is a continuous demand for improved covering materials for fixing members. Various improvements in silicone rubbers have been proposed. See, e.g., Japanese Patent Application Nos. 56-102880; 62-256863, 62-255g82 and 62-252457.
Although it has been possible to considerably lengthen the life of silicone rubber covering by continuously providing the covering with a thin layer of silicone oil at the fixing device during use, the disadvantage of this solution, however, is that silicone oil is transferred to the receiving material so that its writability deteriorates. In image-forming processes in which a powder image is transferred by pressure from an image registration element to the silicone rubber covering of a fixing member, the application of silicone oil cannot be carried out because the image registration element would be soiled by the oil and would become unsuitable for further use.
Accordingly, it is an object of the present invention to provide an improved fixing device. It is a further object of the present invention to provide a fixing member having an improved surface layer of silicone rubber.